1. Field of the Invention
The present invention relates to a physical quantity sensor, and more particularly to the configuration of the output level conversion circuit of a physical quantity sensor.
2. Description of the Related Art
Today, various types of physical quantity sensors are used. Especially, many proposals are made for the correction of the sensor output of an angular rate sensor typified by a vibratory gyroscope (gyro).
The prior-art technology disclosed in Patent Document 1 proposes a method for changing the detection sensitivity of a physical quantity sensor in proportion to a change in the power supply voltage at which the physical quantity sensor operates. This method is known as ratiometric. FIG. 14 and FIG. 15 are diagrams showing a general ratiometric configuration. In this ratiometric configuration, a sensor 110 and an A/D converter 120 receive the supply of the common power voltage Vref. FIGS. 15A-15C and FIGS. 15D-15F show cases in which only one of the sensor 110 and the A/D converter 120 corresponds to the power supply voltage Vref.
FIGS. 15A-15C show an example in which only the sensor 110 corresponds to the power supply voltage Vref. In this example, the output of the sensor 110 (FIG. 15A) is decreased by a change (a decrease in this example) in the power supply voltage Vref. When the output of the sensor 110 is converted by the A/D converter 120 to output digital signals, there is a difference in the A/D-converted digital values because the A/D converter 120 does not correspond to the change in the power supply voltage Vref (FIGS. 15B and 15C).
FIGS. 15D-15F show an example in which only the A/D converter 120 corresponds to the power supply voltage Vref. In this example, the output of the sensor 110 (FIG. 15D) does not depend on a change in the power supply voltage Vref. When the output of the sensor 110 is converted by the A/D converter 120 to output digital signals, there is a difference in the A/D-converted digital values because the A/D converter 120 corresponds to the change in the power supply voltage Vref (FIGS. 15E and 15F).
On the other hand, FIGS. 15G-15I show an example in which both the sensor 110 and the A/D converter 120 correspond to the power supply voltage Vref. In this example, the output of the sensor 110 (FIG. 15G) is decreased by a change (a decrease this example) in the power supply voltage Vref. When the output of the sensor 110 is converted by the A/D converter 120 to output digital signals, there is no difference in the A/D-converted digital values because the A/D converter 120 also corresponds to the change in the power supply voltage Vref (FIGS. 15H and 15I).
The conventional physical quantity sensor shown in FIG. 16 has a configuration in which the output signal of a sensor element 1 detected by a detection circuit 2 is amplified by an amplifier circuit 6.
The amplifier circuit 6 is an inverting amplifier circuit built by an operational amplifier 4 with a MOS device 7 as the input resistor and with a resistor element 8 as the feedback resistor. Biasing the gate voltage of the MOS device 7 with the voltage that varies according to the power supply voltage of the physical quantity sensor allows the detection sensitivity of the physical quantity sensor to be adjusted, especially, the detection sensitivity of the physical quantity sensor to be changed in proportion to a change in the power supply voltage.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-53396 (pp. 4-6, FIG. 1)
However, it is known that the resistance component of the MOS device 7, which generally has non-linearity characteristics, operates as a linear resistor element only when the input voltage is extremely low. Therefore, when the amplitude of an input signal to this amplifier circuit 6 is high, the gain differs between a range where the output signal from the sensor element 1 is high and a range where the output signal is low. This difference in gain does not ensure the linearity of the detection sensitivity (scale factor) of the physical quantity sensor with the result that the ratiometric characteristics remain unimproved.
In addition, depending upon the characteristics of the sensor element or the output characteristics required for the physical quantity sensor, not only the linearity between the power supply voltage and the detection sensitivity but also a desired relation between them is required. However, the amplifier circuit described above, where the MOS device 7 is used, does not give desired sensitivity characteristics because the gain depends on the characteristics of the MOS device.